Electrical double-layer capacitors are currently employed as backup power supplies for computer memories. These capacitors, which make use of the electrical double layer that forms at the interface between an electrode and a liquid electrolyte, have a small size, a large capacitance and a long cycle life.
Recent rapid advances in portability and cordless features in consumer electronic devices such as mobile phones have led to a heightened demand for electrical double-layer capacitors. In particular, electrical double-layer capacitors which use nonaqueous electrolytes have a higher voltage and energy density than those made using aqueous electrolytes. Such capacitors are viewed as holding special promise for backing up power in various types of electrical and electronic equipment, power regeneration in transport devices such as electric cars, and power storage, and have thus been the object of accelerated research efforts.
Secondary cells were once used for such applications. However, electrical double-layer capacitors have come into widespread use as lower power consumption by electronic equipment has reduced backup current requirements, and because of the longer cycle life and broader service temperature range of the capacitors themselves.
Such electrical double-layer capacitors have a construction in which a separator lies between a pair of polar electrodes. The separator is generally impregnated with an electrolyte solution. The polar electrodes have been built by adding to a binder a high surface area material such as activated carbon and an electrically conductive material for improving electrode conductivity, preparing the mixture as a slurry, then coating the slurry onto a metallic current collector such as aluminum foil so as to support the slurry components.
Examples of binders that have been used in electrical double-layer capacitors to support the high surface area material such as activated carbon, the conductive material and other components on the metallic current collector include polytetrafluoroethylene (PTFE), polyvinylidene fluoride, polyvinyl pyrrolidone and carboxymethylcellulose. Of these, polyvinylidene fluoride has excellent film-formability.
However, none of these binders has the ability to dissolve ion-conductive salts to a high concentration or itself possesses a high ionic conductivity. Even were such a binder to dissolve an ion-conductive salt to a high concentration, it would crystallize, preventing the free migration of ions. In addition, to lower the interfacial resistance between the polar electrodes and the electrolyte (separator), it is desirable for a binder resin sharing some of the same components as the electrolyte (separator) to be used as the binder in the polar electrodes.
It is apparent from the above that the performance of binder resins for polar electrodes and the electrolyte compositions for electrical double-layer capacitors hitherto described in the literature leaves something to be desired. A need has thus been felt for further improvement.